1. Field of the Invention
The present invention relates to a method for frequency conversion in which a first signal with a first frequency is converted into a second frequency through mixing with a divided oscillator signal and wherein the frequency of the divided oscillator signal stands in a fractional rational ratio to the frequency of the undivided oscillator signal.
In addition, the present invention relates to a receiver, having at least a mixer, an oscillator, and a switchable frequency divider for dividing a frequency of the oscillator by various natural numbers n, m, wherein the mixer mixes a first signal that has a first frequency with a divided signal from the oscillator and thus converts it to a second frequency, and having a control unit that, in a first operating state, periodically switches the switchable frequency divider from a division by the number n to a division by the number m according to a predefined time-slot pattern.
2. Description of the Background Art
A conversion of a first frequency to a second frequency is customary in, for example, receiving systems for radio frequencies. To this end, both the signal at the first frequency and the output signal of the phase-locked loop are supplied to a mixer that outputs as a result the signal at the second frequency (intermediate frequency).
In special applications it is desirable to be able to set a division factor of 1.5. Division by 1.5 corresponds to multiplication by a factor of ⅔, which means that two output pulses are generated from every three input pulses.
It is known to achieve frequency conversion with a fractional rational frequency ratio by division using the fractional-N principle. This principle is used, for example, in phase-locked loops with fractional rational division ratios to convert an oscillator frequency to a reference frequency. Conventional fractional-N dividers generate a fractional rational frequency ratio by periodically removing pulses from a periodic pulse sequence. Conventional fractional-N frequency dividers thus ultimately suppress output pulses in order to express the fractional rational frequency ratio. This creates an asymmetry in the time behavior of the output signal of the phase-locked loop that is associated with a DC component in the output signal. In other words, the average value over time of the output signal then does not correspond to half of the separation of its extreme values. However, a signal that has a DC component is not suitable for operating a mixer. In the prior art, the DC component must therefore be removed by filtering, which makes the preparation of a signal for the mixer complicated.